1. Field of the Invention
This invention generally relates to creating images of integrated circuits, and more specifically, the invention relates to creating emission images of areas of integrated circuits. Even more specifically, an embodiment of the invention relates to creating high resolution emission images of areas of integrated circuits that do not fit in the field of view of the optical system used for acquiring the images.
2. Background Art
Time-integrated images (also called static images) of the intrinsic near infrared emission from integrated circuits (ICs) is widely used to characterize and test VLSI circuits. For example, Light Emission From Off State Leakage Current (LEOSLC) can be measured by static and dynamic photodetectors such as Charge Coupled Device (CCD) cameras and Photo Multiplier Tubes (PMTs). This emission has found applications in, for example, circuit logic state mapping, power grid drop calculation, circuit internal temperature and gate self heating measurements, and across chip performance variation evaluation. More recent applications of both time-integrated and time-resolved emission images of ICs relate to the detection of changes and alterations of ICs. In general the changes may be wanted or unwanted changes. Circuit design changes or manufacture process changes aimed to improve some measurable characteristics of the IC (such as maximum operating frequency, power consumption, reliability, etc.) are examples of wanted changes. Emission-based techniques may be used to verify and characterize these changes. An example of unwanted changes is the intra-chip and inter-chip variation due to manufacturing process variability such dopant fluctuations of transistor thresholds, line edge roughness of transistor dimensions. Another example of unwanted changes pertains to security applications, where alterations of the IC may cause undesirable changes in its behavior. In this case, emission-based methodologies may be used to localize, identify and characterize these changes that may or may not be observed during electrical testing and diagnostics of the IC.
For many types of applications, images of the entire chip are necessary or useful for extracting important information such as the across chip variability maps. However, the continuous need for higher performance and additional chip functionality with lower costs has resulted in aggressive scaling of transistor dimensions as well as an increase of chip area. In this type of application, it is very important to be able to observe very large areas (possibly the entire chip) with a high spatial resolution that allows to identify the changes in the circuit. This poses a challenge to emission acquisition since cameras have a limited number of pixels and therefore the spatial resolution is inversely proportional to the field of view of the optical system. Specifically, if the magnification of the optical system is increased to obtain a suitable spatial resolution, the field of view is inevitably reduced as a side effect. In modern IC design where gates have sub-micron dimension and chip may be 1 in×1 in in size, a compromise has to be found between area coverage and spatial resolution. In current high end products, manufacturers have already reached the point that the emission from the entire chip cannot be acquired with a single acquisition even at the lower possible magnification offered by most optical system/microscopes.